FIG. 7 is a flowchart illustrating a sequence for aligning a wafer and for exposure in a semiconductor exposure apparatus using an ordinary stage control apparatus. Ordinary stage control in the exposure operation of the semiconductor exposure apparatus will be described using the flowchart of FIG. 7.
First, a wafer is carried into the semiconductor exposure apparatus (step S601). When the wafer is supplied to and placed on an XY stage, the wafer is aligned (alignment processing is executed) with respect to a mask before exposure is carried out (step S602). In this alignment processing, first, approximate alignment of the wafer is performed initially and then the coordinate position of each shot on the wafer is measured.
Next, the mask and wafer are aligned based upon the coordinate position of each shot obtained by the alignment processing and the mask pattern is exposed on the wafer (step S603). At the time of such exposure, displacement of the wafer in the vertical direction is detected in real time and focus control is performed in such a manner that the mask pattern will be transferred to the wafer appropriately. When exposure of the wafer ends, the wafer is transported from the XY stage (step S604). Thus, the sequence of FIG. 7 is repeated by a single wafer stage in an ordinary semiconductor exposure apparatus.
There is also a semiconductor exposure apparatus equipped with a structure, which is referred to as “twin stages”, having two movable stage on which wafers are placed and moved (see the specification of Japanese Patent Application Laid-Open No. 2002-280283). By employing twin stages, throughput can be improved by executing parts of the above-described sequence in parallel. For example, if exposure processing is executed by performing alignment processing and focus measurement using one stage (referred to as a “measurement stage” below) and performing exposure processing using the other stage (referred to as an “exposure stage” below), overall wafer treatment efficiency of the apparatus can be improved. The reason for this is that in the case of twin stages, although the time required for treating one wafer is lengthened by the amount of time it takes to move the wafer between the stage, the time required for alignment processing and focus measurement generally is longer than the time it takes to move the wafer. Further, with the twin stages, information relating to unevenness of a wafer surface measured beforehand by the measurement stage can be used at the time of exposure processing at the exposure stage and this makes it possible to obtain excellent focus follow-up. More specifically, a focus target value is created based upon information related to wafer-surface unevenness obtained by measurement at the measurement stage, and the position of the exposure stage along the optical axis is controlled based upon this focus target value, thereby implementing focus tracking that conforms to wafer unevenness.
An ordinary exposure apparatus having twin stages only uses the focus target value as one control target value in a stage apparatus having a feedback system. Consequently, if the wafer surface has a great amount of unevenness or the pattern used has a large step, the stage control system cannot follow up and there is the possibility that the desired focusing ability will not be achieved.